I. Field of the Invention
The invention deals with the removal of metals from compositions containing such metals as organic or inorganic compounds. It more particularly refers to removal of lead from gasoline.
The need for the removal of metals is evidenced by the fact that their presence in hydrocarbon charge stocks conducted to catalytic cracking and catalytic reforming process units is known to poison and shorten the life of the catalyst with which such metal contaminated stocks come into contact.
It is also desirable to remove trace metals from lubricating oils or to recover soluble metal catalysts from reactor effluents or polymer solutions. The removal of heavy metals such as mercury, silver, calcium and the like from the water effluents of chemical or photographic plants is also highly desired from an ecological standpoint.
Unburned hydrocarbon, carbon and carbon monoxide emissions are regarded by many as representing a substantial source of air pollution. These engine emissions are subject to photo-chemical reaction in the atmosphere, providing what has been termed smog, which is an irritant of lachrymal and respiratory system tissues.
The incompletely oxidized carbon, e.g., carbon monoxide and unburned hydrocarbons, present in engine exhaust is the result of incomplete combustion of the hydrocarbon fuel in the engine combustion chamber. Complete oxidization of such carbon monoxide and/or hydrocarbons transforms such to carbon dioxide and water, probably in the form of steam due to the high combustion temperature. Neither carbon dioxide nor steam is considered a harmful emission.
Various means have been employed to reduce or eliminate carbon monoxide and unburned hydrocarbon emissions. One approach has been to pass the combustible exhaust gases through a catalytic converter located in the engine's exhaust system where the carbon monoxide and hydrocarbons are catalytically oxidized, usually by the introduction of supplemental air, to carbon dioxide and water.
It is known that residues of alkyl lead from combustion of leaded gasoline tend to poison catalysts available for oxidizing unburned hydrocarbons and carbon monoxide in an engine exhaust. Such poisoning severely shortens the useful life of exhaust combustion catalysts. Furthermore the presence of lead complexes in engine exhaust is considered objectionable by some. Therefore the removal of lead from gasoline might be desirable separate and apart from its characteristic of poisoning exhaust combustion catalysts. It has thus been heretofore proposed that "lead free" gasoline be supplied for use in automobiles equipped with emission control devices which utilize catalysts to help further oxidize exhaust gases.
Under most proposals, small, trace amounts of lead would be allowed in "lead free" gasoline. The Federal Government regulations require all gasoline sales outlets to furnish at least one grade of gasoline having less than 0.07 gram of lead per gallon to the public by July 1, 1974. On Feb. 23, 1972 the Environmental Protection Agency in a paper relating to the 1970 Clean Air Act Amendments offered for comment a requirement of 0.05 grams of lead per gallon of gasoline. The lead level reduction is to be accomplished by July 1, 1974. Other proposals have been even more stringent.
The normal network of petroleum product distribution involves railroad tank cars, pipelines, water borne tankers, tank trucks and bulk storage tanks. For commercial operation these are presently set up to handle different products. For example, the same pipeline might be used to convey a shipment of regular grade gasoline, premium grade gasoline, distillate fuel and other light liquid products in succession. According to present procedures, that portion of the fluids carried by the pipeline which constitutes an intermingling of the two products at their interface is diverted to use with the lower grade product, thus avoiding degradation of the higher grade hydrocarbon.
However when leaded gasoline is followed by leadfree gasoline, not just the interface comprising in intermingling of the two products, but the entire leadfree shipment becomes degraded. When leaded gasoline, containing tetraethyl lead, tetramethyl lead or a mixture of transalkylation products of the two is contacted with the metal or other surfaces of transportation and storage facilities, a significant amount of lead is left deposited in scale and on these surfaces. Since alkyl lead components are infinitely soluble in light hydrocarbons such as gasoline, upon after using the same faculties for lead free gasoline the latter product becomes contaminated with lead which may run as high as about 0.1 grams of lead per gallon or more. These amounts of lead are sufficient to impair the life of exhaust emission control oxidation catalysts and are in excess of the presently proposed allowable limits on lead content of lead free gasolines.
II. Description of the Prior Art
Techniques have heretofore been known for removal of dissolved or suspended heavy metal contaminants from liquid products.
In catalytic cracking and reforming operations, the use of guard chambers containing a variety of sorbents and/or catalysts intended to remove heavy metal contaminants from the charge stock before contact is made with the catalyst have been described. Catalytic hydrodesulfurization processes and catalysts remove some amount of heavy metal contamination from hydrocarbon streams processed thereby.
Systems for removal of lead from gasoline have also been proposed. Presently known techniques require considerable time or are non-selective in effecting removal from the gasoline not only of the lead but also of those additives which are desired to be retained, such as antioxidants, anti-icing additives, metal passivators, detergents and the like.
One previously proposed system for removing lead from gasoline is described in U.S. Pat. No. 2,368,261. There, acid activated clay, such as bentonite which had been previously treated with hydrochloric or sulfuric acid, is used. Leaded gasoline is percolated through the clay whereby up to 95% of the lead present is removed. Unfortunately, acid activated clays will also remove other gasoline additives which are required or desired for proper protection and functioning of automotive equipment.
Another approach is that described in U.S. Pat. No. 2,392,846. According to an Example in this patent, a five gallon lot of leaded gasoline is treated with 20 ml. of stannic chloride followed by addition of 100 grams of activated carbon. This results in decomposition of the tetraalkyl lead and adsorption of the lead decomposition products on the activated carbon thus drastically reducing the lead content. The gasoline is removed from the activated carbon by decantation. This is a very slow process which permits the processing of about 35 gallons of gasoline per hour. Unfortunately even in this system, the additives desired to be retained in the gasoline are also adsorbed by the activated carbon.
Both the processes described in the cited prior patents depend for effectiveness on a chemical conversion of the tetraalkyl lead. The lead compounds can be reacted with such materials as halogens, halogen acids, metal halides, metal salts, sulfur dioxide, carboxylic acids, metals in the presence of hydrogen etc. While alkyl leads are infinitely soluble in gasoline, the resulting decomposition products are not readily soluble in hydrocarbons and hence can be selectively adsorbed on high surface adsorbents.
The American Oil Company, in a paper presented at the May 9, 1972 meeting of the API Division of Refining noted that in a significant number of its stations it was presently unable to meet the 0.05 gram/gal. or even the 0.07 gram/gal. requirement using scrutinous control of their distribution system and segregation of products. The area of greatest potential contamination was that of the service station itself. The report would indicate that all gasoline manufacturers relying only on distribution control to ensure that the unleaded gasoline will remain within specifications, face an extremely difficult and expensive undertaking.
It is a primary objective of this invention to provide means to remove heavy metals from liquid, particularly hydrocarbon, streams. It is a further objective of this invention to remove lead alkyls from gasoline. It is an objective of this invention to selectively remove these metals from hydrocarbon streams by such means as will not remove gasoline additives, such as detergent additives, from gasoline streams. It is an objective of this invention to provide for means for removing lead alkyls from gasoline, such means being capable of regeneration. Other and additional objects of this invention will become apparent from a consideration of this entire specification including the claims and drawings.